Test independence in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why is test independence important in JUnit?

In JUnit testing, why should each test method be independent from others?

A To reduce the total number of test methods needed

B To ensure tests can run in any order without affecting results

C To allow tests to share state for faster execution

D To make tests dependent on the success of previous tests

💡 Hint

Think about what happens if tests rely on each other's data or state.

Test independence ensures that tests do not affect each other, so they can run in any order and still produce reliable results.

❓ Predict Output

intermediate

Output of dependent JUnit tests

Consider these two JUnit test methods. What will be the output if testB runs before testA?

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

public class SampleTest {
    private static int counter = 0;

    @Test
    public void testA() {
        counter = 5;
        assertEquals(5, counter);
    }

    @Test
    public void testB() {
        assertEquals(5, counter);
    }
}

A Both testA and testB pass

B Both testA and testB fail

C testA fails because counter is not set, testB passes

D testB fails because counter is 0, testA passes

💡 Hint

Remember static variables keep their value across tests unless reset.

testB expects counter to be 5 but it is 0 initially. Since testB runs first, it fails. testA sets counter to 5 and passes.

❓ assertion

advanced

Correct assertion for independent test

Which assertion best ensures that a JUnit test is independent and does not rely on previous test state?

A assertTrue(sharedFlag) set by another test

B assertNotNull(sharedObject) without initialization in this test

C assertEquals(expectedValue, actualValue) with setup in @BeforeEach

D assertEquals(0, counter) relying on static variable from previous test

💡 Hint

Think about where the test data comes from and if it is initialized fresh each time.

Using @BeforeEach to set up expected values ensures each test runs with fresh data, making it independent.

🔧 Debug

advanced

Identify the cause of flaky JUnit tests

These JUnit tests sometimes fail and sometimes pass. What is the most likely cause?

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;
import java.util.ArrayList;
import java.util.List;

public class FlakyTest {
    private static List<String> sharedList = new ArrayList<>();

    @Test
    public void testAdd() {
        sharedList.add("item");
        assertEquals(1, sharedList.size());
    }

    @Test
    public void testClear() {
        sharedList.clear();
        assertEquals(0, sharedList.size());
    }
}

A Tests share mutable static state causing order-dependent failures

B Tests use different assertion methods causing conflicts

C Tests have syntax errors causing random failures

D Tests run in parallel causing thread safety issues

💡 Hint

Look at how the sharedList is used and modified.

Using a static shared list modified by tests causes flaky results depending on test execution order.

❓ framework

expert

Best practice to ensure test independence in JUnit 5

Which JUnit 5 feature helps guarantee test independence by resetting state before each test method?

A @BeforeEach annotated method to initialize fresh state

B @TestMethodOrder to run tests in a specific sequence

C @BeforeAll to set up shared state once for all tests

D @RepeatedTest to run the same test multiple times

💡 Hint

Think about how to prepare the environment freshly for each test.

@BeforeEach runs before every test method, allowing fresh setup and ensuring tests do not share state.